[Elecraft] My KAT500 seems very sensitive to common mode RFI

[Don Wilhelm]

Jeff and all,

I see many hams reporting placing common mode chokes on AC lines, DC 
lines, AUX cables and other lines in the shack.

I am of the opinion that the proper use of common mode chokes is on the 
antenna feedlines.  With coax fed antennas, a choke right at the 
feedpoint and with parallel line non-resonant antennas, at the point of 
transition from parallel line to coax.  You may not notice a problem at 
lower power, but if the power is increased to 500 watts (or to the legal 
limit), problems not apparent at lower power begin to 'raise their ugly 
heads'.

Chokes placed in the antenna system locations will reduce the amount of 
RF getting back into the shack and will reduce the need for chokes on 
the shack lines themselves.

Of course, if the antenna radiation is coming directly into the shack 
(such as from an antenna in close proximity to the shack), then you may 
still need additional chokes in the shack.

Those using offset center fed dipoles or end fed dipoles will have to 
take more extreme measures (use the K9YC common mode chokes at the 
antenna feedpoint and at the feedline entry point to the shack).  Those 
antennas are 'infamous' for creating 'RF in the Shack'.  It may not be a 
problem at 100 watts, but the problem grows exponentially as the power 
is increased.

73,
Don W3FPR

On 2/14/2015 9:46 PM, Jeff Hall wrote:
> I have several turns of the AC adapter cord through two snap on beads.   I don't have anything on the AUX cables at this time, but that is something I should probably check.   I ordered 6 of the Mix 31 Big Clamp toroids from KF7P.  For the 40m vertical I'll place one choke at the feedpoint and see if the problem is resolved.  If not, I'll place another choke at the KAT500.  If I still have a problem, another choke will be placed at the output of the KPA500.  If that still doesn't work, another choke will be placed at the output of the K3.  Once I have suppressed the common mode RF on the vertical, I'll try replacing the string of ferrite beads on my hex beam's coax with a Big Clamp choke at the KAT500.
> -Jeff W6UX
>